The puppies I can think of being advertised this way by people I know are because they are good puppies that the breeder will hold back until a good home is found. They are in no rush to sell/move the puppy and feel the dog has good potential. Why the breeder makes this designation can be many things. Maybe the puppy is too strong or too sharp for a pet home or an experienced owner, maybe the breeder just wants to ensure that some of their puppies end up competing and earning titles.